There where many reasons why SEGA picked up Atlus, personally think one of the reasons was how well they did during the last console generation, especially when you consider the size of the publisher. Its not surprising that Atlus is being rumored as to bringing back a popular title from the PlayStation 3 era.
According to the report, this would be a sequel and/or continuation of the franchise in some sort of way. According to the source, there is also a chance that this could be a port of that PlayStation 3 game with extra content (see Dragon Crown Pro). The source also mentions that this title isn’t a numbered title and didn’t receive any DLC when it came out. Another hint in this report was that the title was teased a few months back.
So, what would the most educated guess be? Catherine! Catherine was a important title for Atlus in the PlayStation 3 era, I remember it was one of their first titles that I actually saw being sold in a store like Walmart and was a massive hit even tho the game was very niche in gameplay. According to Siliconera, a new Catherine was hinted at months ago, the title was popular and it didn’t have DLC. I think its safe to say its Catherine related. But a new game or a port with extra content? What do you want?